Fipronil [5-Amino-1-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-trifluoro methyl sulfinyl pyrazole] is one of the important fluorine bearing 1-Aryl pyrazole derivatives developed in the recent two decades.

It is a broad spectrum insecticide that disrupts central nervous system of insect by blocking the passage of chloride ions through the GABA receptor and glutamate-gated chloride channels (GluCl).
It is mainly used in agriculture fields. Fipronil is particularly applied to soil and seeds to control insects. e.g. fipronil is used to control i) multiple species of thrips on a broad range of crops by foliar, soil or seed treatment, ii) corn rootworm, wireworms and termites by soil treatment in maize, iii) boll weevil and plant bugs on cotton, iv) diamond-back moth on crucifers, v) colorado potato beetle on potatoes by foliar application, vi) stem borers, leaf miners, planthoppers, leaf folder/rollers and weevils in rice.
Various processes for the synthesis of fipronil are described in the following documents:    EP295117; EP460940; EP484165; EP668269; EP967206; EP1331222; EP1374 061; U.S. Pat. No. 5,631,381; CN1374298; and J. of Heibei University of Science and Technology, Vol. 25 (2), Sum 69 (2004), Dok. Serial No. 1008-1542 (2004) 02-0018-03.
The characterization of fipronil is generally carried out by measurement of melting point and by analysis such as IR spectrum, proton & carbon-13 NMR, HPLC, UV and the like.
Polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Typically, when a compound is re-crystallized from a solution or slurry, it may crystallize with different spatial lattice arrangements. The different crystal forms individually are referred as a “polymorph”. Different polymorphic forms of a given substance may differ from each other with respect to one or more physical properties, such as solubility and dissociation, true density, crystal shape, compaction behavior, flow properties, and/or solid state stability. Different types of crystalline forms typically affects the manufacturing process such as mixture of two different crystalline forms clog the pores of filters which leads to loss of time and product. Further, it requires tedious and expensive cleaning work. Furthermore, different bulk densities affect the storing and packaging requirements.
Various crystalline modifications of fipronil are disclosed in the following patent documents:
WO2007069254 discloses crystalline polymorph Form I of fipronil which exhibits an X-ray powder diffraction pattern having characteristic peaks expressed in degrees 2θ(+/−0.20° θ) at about 13.04, 16.27, 18.48, 19.65, 22.05, and 31.55. It also discloses crystalline polymorph Form II of fipronil which exhibits an X-ray powder diffraction pattern having characteristic peaks expressed in degrees 2θ(+/−0.20° θ) at about, 14.4, 15.7, 16.75, 17.2, 19, 20.7, 22.95, 23.55, and 32.15.
WO2007069254 also discloses preparation of two types of crystalline polymorphic forms of fipronil. Initially fipronil Pseudomorph is prepared by dissolving fipronil in toluene followed by heating upto 110° C. The obtained Pseudomorph is heated upto 150° C. to form fipronil form III which on exothermic transition get converted to form I. fipronil form II is prepared by dissolving fipronil in isopropyl alcohol or n-hexane or methyl isobutyl ketone followed by heating upto 95° C.
WO2008055881 discloses a crystalline modification I of fipronil showing, in an X-ray powder diffractogram at 30° C., at least 5 of the following reflexes: (1) d=7.45±0.1 A, (2) d=6.07±0.07 A, (3) d=5.57±0.05 A, (4) d=4.84±0.05 A, (5) d=3.76±0.05 A, (6) d=3.67±0.05 A (7) d=3.23±0.05 A, (8) d=3.01±0.05 A and (9) d=2.77±0.05 A. It also discloses preparation of crystalline polymorphic form of fipronil. Fipronil is dissolved in a solvent such as methanol, isopropanol, ethanol, ethyl benzene, diisopropyl benzene, n-butyl benzene, CF3-benzene, acetonitril, DMSO followed by heating.
WO2008055882 discloses a crystalline modification II of fipronil which has an X-ray powder diffractogram showing, at 25° C., at least 5 of the following reflexes: (1) d=13.44±0.2 A, (2) d=7.84±0.1 A, (3) d=5.50±0.07 A, (4) d=5.14±0.05 A (5) d=4.95±0.05 A, (6) d=3.95±0.05 A, (7) d=3.77±0.05 A, (8) d=3.22±0.03 A and (9) d=2.91±0.03 A.
It also discloses a process for preparing the crystalline modification II in which fipronil is dissolved in a solvent selected from tetrahydrofurane, 1,2-dichloroethane, acetonitrile, mono-, di- or tri(Ci-C6-alkyl) benzenes followed by heating upto 137° C.
WO2008055883 discloses a crystalline modification V of fipronil showing, in an X-ray powder diffractogram at 30° C., at least 5 of the following reflexes: (1) d=8.55±0.1 A, (2) d=7.94±0.07 A, (3) d=6.78±0.05 A, (4) d=5.43±0.05 A (5) d=4.35±0.05 A and (6) d=2.83±0.03 A.
It also discloses a process for preparing the crystalline modification V in which a solution of a solid form of fipronil is prepared by dissolving it in a solvent such as dimethylsulfoxide or acetonitrile followed by heating at 60° C. to 150° C.
WO2008055884 discloses a crystalline modification IV of fipronil which has an X-ray powder diffractogram showing, at 30° C., at least 5 of the following reflexes: (1) d=11.28±0.2 A, (2) d=9.04±0.1 A, (3) d=7.61±0.07 A, (4) d=6.46±0.05 A, (5) d=5.28±0.05 A, (6) d=4.59±0.05 A, (7) d=3.59±0.03 A, and (8) d=3.04±0.03 A, It also discloses a process for preparing a crystalline modification IV of fipronil in which a solid form of fipronil is dissolved in acetone followed by crystallization and isolation.
The processes disclosed in the prior art are carried out at higher temperature. Thus there is an urgent and unmet need in the art for an efficient method for the preparation crystalline fipronil, which is simple and can be used on a large scale for industrial manufacture and which produce highly pure product that can be safely utilized.